Process for the production of a titanium alloy based composite material reinforced with titanium carbide, and reinforced composite material obtained thereby

ABSTRACT

Object of the present invention is a process for the production of a Titanium alloy based composite material with satisfactory mechanical features at high temperature, characterised in that Titanium alloy powders and Titanium carbide powders are blended, hot-pressed and hot-rolled or extruded. The invention also encompasses a composite material obtainable with said process.

The present invention refers to the production of components obtained from Titanium alloy based components, to be used in the field of mechanics and of high temperature automotives in the presence of creep and of high specific stresses.

EP-0 215 941 (Dynamet) teaches the manufacturing, by blending and sintering, of Titanium-based composite materials including a dispersion of Titanium carbide (TiC) powder. The end product obtained is free of a significant reaction at the TiC-matrix interface or of dilution regions exhibiting a composition gradient.

The main restriction of this US process is that the product obtained has an interface exhibiting scarce chemical reaction, and therefore where the stresses are accordingly transferred by mechanical mechanisms. Moreover, exposure to high temperatures fosters grain growth, a phenomenon that worsens the mechanical properties, especially the fatigue strength.

Therefore, in the specific field there subsists the demand for a manufacturing process allowing to overcome the abovementioned drawbacks.

The process subject of the present invention surmounts all of the abovementioned drawbacks, further providing other advantages that will be mentioned hereinafter.

In fact, object of the present invention is a process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature, wherein a Titanium alloy powder and a Titanium carbide powder are blended, hot-compacted and hot-rolled or extruded.

The TiC content (concentration) can range from 0.5 to 30% b/w.

The granulometry of the Titanium alloy can be <250 μm, preferably <5 μm.

The blending of the two powders may be carried out in the presence of the 50% b/v acetone (or other anti-clumping agent), optionally added separately to each one of the powders to be blended, prior to blending.

The powder blending may be carried out under inert gas, e.g., Argon, atmosphere.

The blending may be obtained by revolving in a vessel, containing the two powder types, at a high rate of rpm for a time ranging from 5 minutes to 8 hours.

The hot compacting may be obtained by hot isostatic pressing (HIP) at temperatures ranging from 850 to 950° C., at pressures ranging from 80 to 130 MPa, for <4 h times.

The powders thus blended can be dried substantially under vacuum.

The material resulting from the hot pressing is preferably heated to a temperature of about 1000° C. and pressed to a thickness reduction of from 5 to 50%.

The yielded pressed product is rolled, at temperatures comprised in the range 800-1000° C., with <5% reduction passages, down to the desirable total thickness reduction, e.g. of about 80%.

The process according to the present invention allows an optimum distribution of the TiC particulate and the diffusion thereof at the interface with the Titanium alloy matrix.

The diffusion is measured from a Carbon (C) content of about 20% at 20 μm from a TiC particle.

The carbon diffusion obtained by TiC particles/Titanium alloy matrix interface reaction is controlled via the thermal treatment of hot compacting.

In particular, in order to obtain a C content of about 17% in atom percent, at a 20-μm distance from a TiC particle, a 960° C. temperature should be applied for 3 h with pressures of about 1100 MPa.

This significant dissolving of the TiC inside of the Titanium alloy matrix is accountable for the increase of the mechanical properties attained with the present invention.

With respect to the composite material disclosed in EP 0 215 941, for the composite material of the present invention the rolling step advantageously suffices to eliminate Titanium carbide agglomerates; these carbides, very uniformly dispersed, allow to overcome brittleness problems at room temperature, with breakaway of the bonds at the old particle edges in the unrolled material. The measured strength values are about 20-30% higher than those of the composite alloy of EP 0 215 941. This advantageous result could also be accounted for by the fact that inside of the matrix an evident dilution of the TiC has occurred, with a C concentration profile that drops from about the 50%, measured at the centre of a TiC particle, and stabilizes, after about 20 μm, to values of about 5%, measured also at about 60 μm from the edge of the TiC particle.

The Titanium alloys that yielded satisfactory results as matrices in the compounds according to the present invention are the following. Ti6Al4V, Ti6Al2Sn4Zr2Mo0.1Si, Ti15Al3V3Sn3Cr, and Ti6242S.

The present invention also refers to the composite material obtainable with the hereto-described process.

So far, a general description of the present invention was given. With the aid of the attached figures and of the following example, a more detailed description of specific embodiments of the invention, aimed at making better understood the objects, the features, the advantages and the operation modes thereof will be provided hereinafter.

FIG. 1 shows the microstructure of an embodiment of the homogeneous blend of Titanium alloy powder and TiC powder, prior to the hot compacting.

FIG. 2 shows the increase of the mechanical properties, at <600° C. temperatures, of a compound obtained with an embodiment of the process according to the invention with respect to the material obtained with the same powders by sintering and hot compacting.

EXAMPLE

The powders to be blended according to the invention are prepared by gas atomizing from 500 mm high, 45 mm Ø ingots. The end sizes of the particles obtained are <200 μm for the Ti6242S alloy, utilized in the example, and <10 μm for the Titanium carbide.

Table 1 shows the composition of the Titanium alloy powder with respect to that of the starting ingot. The Table also reports the average size (in μm) of the Ti6242S powder, the flow rate and the size (in μm) of the TiC particles. TABLE 1 Al Sn Zn Mo O N H Ti6242S Alloy % % % % ppm ppm ppm Pre-atomizing samples 6.1 1.4 3.7 1.6 646 218 nd Powder samples 6.3 1.3 3.8 1.7 1096 496 90 Average size (μm) Flow rate TiC particle size of Ti6242S particles (ASTMB213) (μm) 44-200 28s 0.1-5

Ti6242S and TiC powders are blended in a rotary cylinder with movable blades, instead of resorting to a mechanical alloying that produces more superficial fractures and, therefore, more reaction sites with C, O, N. This procedure, as well as the mechanical alloying, provides optimum reinforcing material/matrix homogeneousness. FIG. 1 shows the 200×SEM (Scanning Electron Microscopy) microphotography of the homogenate blend.

Then, the blended powders are introduced in a steel cylinder that is sealed and welded to the lid by TIG (Tungsten Inert Gas) welding. The cylinder lid is provided with a port and a piping for carrying out the evacuation. The cylinder-shaped container was designed in order to resist fractures during the HIP process.

The evacuation takes place with a rotary pump, obtaining vacuums in the order of 10⁻⁵ mbar.

Post-evacuation, the powder is isostatically pressed, with ho prior consolidation, for 5 h at a 1000° C. temperature and with a pressure peak of 1500 Bar.

Then, tensile test samples of the yielded composite material are obtained, with their axes parallel to the cylinder generatrix.

After heating to 1100° C., the composite material is hot-rolled, with an 80% thickness reduction.

Samples of the rolled material thus obtained are subjected to tensile tests. The test results highlight an increase of the tensile strength of the material at <600° C. temperatures and a decrease of this parameter at >600° C. temperatures. FIG. 2 shows the increase of the mechanical properties of the rolled composite material of the invention with respect to that of the material merely sinterized and compacted by HIP. 

1. A process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature, in which a Titanium alloy powder and a Titanium carbide powder are blended, hot-compacted and hot-rolled, or extruded, characterised in that the hot compacting is obtained by isostatic hot pressing at a temperature ranging from 850 to 950° C., at a pressure ranging from 80 to 130 MPa, for a time less than four hours, and the resulting material is heated to a temperature of about 1000° C. and pressed to provide a thickness reduction of from 5 to 50%.
 2. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 1, wherein the concentration of the Titanium carbide expressed in percent by weight ranges from 0.5 to 30%.
 3. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 1, wherein the particle size of the Titanium alloy is less than 250 μm.
 4. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 3, wherein the particle size of Titanium carbide is less than 5 μm.
 5. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 1, wherein the blending of the said two powders is carried out in the presence of 50% by volume acetone or of an anti-clumping agent, optionally added separately to each of the powders to be blended.
 6. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 1, wherein the blending of the two powders is carried out under inert gas, preferably Argon, atmosphere.
 7. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 6, wherein the blending is obtained by revolving a vessel, containing the two powders, at a high rate for a time ranging from 5 minutes to 8 hours.
 8. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 7, wherein the blended powders are dried under vacuum.
 9. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to any of the preceding claims, wherein the resulting compound is hot-rolled in the range from 800 to 1000° C. with less than 5% reduction passages, until the desired total thickness is achieved.
 10. The process for the production of a Titanium alloy based composite material having satisfactory mechanical features at high temperature according to claim 9, wherein the total thickness reduction is of about 80%.
 11. A Titanium alloy based composite material reinforced with Titanium carbide, as obtained by the process of claim
 1. 